According to the Tenth Amendment to our Constitution education is a residual power of the states.

If Pennsylvania does require physical education then there is nothing that you can do about it short of not getting a high school diploma or dropping out of school. It is no big deal to take physical education classes.

Sometimes the federal government withdraws funding to a school district or to a state if they do not abide by the terms of the grant. It may be that your school district has received funding for physical education that might require as one of the conditions for students to take physical education. I do not know this as a fact but this might be interpreted by some as a federal mandate.

http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2006/06/08/BAG46JAKDG1.DTL&type=printable